1. Field of the Invention
The present invention relates to selective catalytic reduction, and more particularly to methods for controlling and diagnosing such catalytic reduction as well as for controlling internal combustion diesel engines attached to such catalyst systems.
2. Background of the Invention
Diesel internal combustion engines typically operate predominantly lean of stoichiometry. As such, various emission control devices have been used to reduce NOx emissions generated by the engine operation. One such device is a lean NOx catalyst. One example of such a catalyst is a selective catalytic reduction (SCR) catalyst that utilizes a reductant, such as ammonia carried in urea, to reduce NOx in an oxygen rich atmosphere. Other lean NOx catalysts utilize different reductant, such as diesel fuel or other hydrocarbons, present in the exhaust gas.
One method for using such a catalyst is described in U.S. Pat. No. 5,891,409. Here, an upstream oxidative catalyst is used to convert NO into NO2 in the present of oxygen. Then, a downstream catalyst converts the NO2 to N2, CO2, and H2O in the presence of injected hydrocarbons.
The inventors of the present invention have recognized a disadvantage with such a system. In particular, the present inventors have recognized that such catalyst systems can be susceptible to sulfur degradation. In other words, sulfur contained in fuel or engine oil can reach the catalyst and cause degradation of the active components of the catalyst. Such degradation can cause increased emissions. Further, the present inventors have recognized that this sulfur degradation is affected by the ratio of NO to NO2 entering the catalyst. In other words, when operating an SCR catalyst downstream of an oxidation catalyst, a mixture of substantially all NO2 can result in rapid sulfur degradation, thereby giving degraded emissions performance.
In one aspect of the invention, the above disadvantages are overcome by a diagnostic system for an exhaust gas system of a diesel internal combustion engine. The system comprises: an upstream oxidation catalyst for converting a portion of incoming NO into NO2; a downstream lean NOx catalyst for converting at least some of said NO and NO2 exiting said upstream oxidation catalyst into nitrogen in the presence of a reductant; a sensor coupled downstream of said lean NOx catalyst; and a controller for determining whether said upstream oxidation catalyst is providing a preselected exiting NO to NO2 ratio of based at least on said sensor and for determining whether said upstream catalyst is within a predetermined temperature range, and providing an indication of degradation of said upstream catalyst based on said determination.
By monitoring whether the preselected ratio is within acceptable limits, for example, within 50% of a 1:1 molar NO to NO2 ratio, it is possible to determine when sulfur degradation can result in the downstream catalyst due to degradation of maintaining the preselected NO to NO2 ratio and a drop in the NOx conversion efficiencies. In other words, the present invention can recognize when the NO to NO2 ratio of the exhaust gas entering the downstream catalyst is such that it reduces NOx conversion and sulfates the downstream catalyst.